The high temperatures experienced by materials in applications such as the iron and steel industry typically requires the used of ceramic composites. Temperatures in excess of about 2200° F. limit the use of most ceramics to a single use due to the high thermal shock experienced by the material and the limitations of the composite structure. Typical applications of ceramic composites in the iron and steel industry include slide gates, tundish lances, and various castable shapes such as cones and mill rolls. Other applications include fuel cells and electric kiln tiles. In another high temperature application, the materials from which rocket nozzles currently are manufactured include polymer matrix composites and carbon-carbon composites. The ablative and erosive characteristics of polymer matrix composites limit nozzle performance, and carbon-carbon composites entail high manufacturing costs and possible environmental dangers. To meet the multiple requirements of high performance such as resistance to erosion, ablation and thermal shock, and low manufacturing cost, a unique approach to nozzle fabrication must be taken.
Methods exist to fabricate ceramic composites for less demanding, lower temperature applications. One process results in an extensively microcracked multi-phase ceramic capable of withstanding severe thermal conditions. Briefly, a woven preform or a felt mat of ceramic fibers may be impregnated by immersion in a pre-ceramic sol and fired. During firing the fibers of the preform react with the colloidal ceramic particles in the sol to produce a ceramic material having compositional gradients essentially normal to the original fiber directions. The extent of reaction between fiber and matrix, which is controlled by temperature and materials selection, may be limited to leave a residual fiber architecture in place. A high degree of microcracking in the fired ceramic can be ensured by the appropriate combination and size of the starting materials. Porosity in the product can be reduced to a desired level by one or more cycles of vacuum re-impregnation and firing. The extensive microcracking imparts some degree of thermal shock resistance to the ceramic. While a residual fiber architecture leaves the ceramic with a crack deflection network, the distribution of micro cracks may provides some additional stress relief mechanisms to prevent macroscopic failures resulting from large and sudden temperature changes.
Some methods of fabrication of ceramic composite may utilize reinforcement such as magnesium silicate glass, wherein upon firing of the composite the reinforcement interact with the sol matrix in such a manner as to be indistinguishable from the matrix, thus limiting the thermal shock resistance of the material. The fibrous matrix may be retained but the temperature limitations of the matrix and fiber still limit the use of the ceramic to applications below about 2200 F. Furthermore, the resulting composite may be deficient in thermal shock resistance and impact resistance for more demanding, high temperature applications.
It is desirable, and therefore an object of the invention, to provide a higher temperature ceramic composite that will resist thermal shock, impact, chemical attack and withstand temperatures of greater than 2200 F.